Overcurrent protection for parallel batteries

[pocim]

Hi everyone,

My name is Pierre-Olivier Cimon and I am the head of electric powertrain of the electric team Formule SAE de l’Université Laval.

I am turning to you, Formula SAE community, because I would like to have a second opinion regarding the rule EV6.1.4 that states: "If multiple parallel batteries [...] are used then each string must have individual overcurrent protection to protect all the components on that string."

After a question I asked to Formula SAE Electric officials, it looks like the applicable interpretation at Lincoln is that EV6.1.4 applies to single cells in parallel even if the rule doesn't mention "single cell" but only refers to “batteries” and “strings of cells”.

To my opinion, this interpretation is problematic for several reasons. First, the rule doesn’t clearly specify that single cells in parallel need to be protected against overcurrent. Second, it is not uncommon in the industry to find single cells in parallel, with no overcurrent protection, to increase the battery pack overall capacity without increasing the voltage. Third, because Formula SAE Electric is the only competition to my knowledge to interpret EV6.1.4 this way, it prevents European electric teams to compete in America. It is worth noting that our team successfully passed all technical inspections at Formula Student Germany, Austria and Hungary in 2015 with a 3P configuration that had no overcurrent protection between the parallel cells.

Please let me know what you think about this.

Best regards,

Pierre-Olivier

[nat45928]

The FSAE judges have interpreted this rule to be a string is one or more cells. They are trying to prevent cheap cells from forming internal shorts and taking out a larger set, leading to thermal runaway at worst. It definitely poses a design challenge but leads to a much safer pack.

If you get a letter from the manufacturer stating that the cell can be used in parallel configurations without fusing they will let you through tech and compete. High quality cells will probably be fine like this, cheaper ones can cause issues.

Nathan